The present invention relates generally to the field of seismic exploration. In one aspect, this invention relates to an apparatus for measuring the actual seismic energy imparted to the earth by a vibrating baseplate. In another aspect, this invention relates to a method for measuring the actual seismic energy imparted to the earth by a vibrating baseplate.
Seismic exploration for subterranean hydrocarbon deposits is common practice in the oil and gas industry. Reflection surveys are the most common type of seismic surveys in practice today. In reflection-type seismic surveys, seismic waves are induced into the earth and reflected back to the surface by subsurface strata. At the surface, the reflected seismic waves are detected by a group of spaced apart receivers called geophones. The geophones produce seismic reflection signals which can be processed to form images of the subsurface. The energy source used to induce the seismic waves in the earth can be impulsive or vibrational. Impulsive seismic sources, such as dynamite, impart a brief, but powerful seismic wave into the earth. In many locations, however, the use of impulsive seismic sources is impractical due to safety and environmental concerns. As a result, vibrational energy sources have become the preferred means for imparting seismic waves in the earth.
The most common vibrational seismic exploration system used today is known commercially as a VIBROSEIS(copyright) system. This type of system typically uses a quasi-sinusoidal reference signal, or so-called pilot signal, of continuously varying frequency, selected band width, and selected duration to control the injection of seismic waves into the earth. The pilot signal is converted into a mechanical vibration in a land vibrator having a baseplate which is coupled to the earth. The land vibrator is typically mounted on a carrier vehicle which provides locomotion. During operation, the baseplate is contacted with the earth""s surface and the weight of the carrier vehicle is applied to the baseplate. A servo-hydraulic piston connected to the baseplate is then excited by the pilot signal, causing vibration of the baseplate against the earth.
Vibrational seismic sources, such as those employed in the VIBROSEIS(copyright) system, are typically much less powerful than impulsive seismic energy sources, such as dynamite. Thus, in order for vibrational seismic sources to impart a signal of sufficient energy for accurate measurement, the duration of the vibrational signal must be longer (i.e., 2 to 20 seconds) than the duration of an impulsive signal (i.e., a few milliseconds). To assure good resolution, the relatively long reflected seismic signal received by the geophones is compressed to a short pulse, similar to that which would be produced by an impulsive seismic system. This principal of xe2x80x9cpulse compressionxe2x80x9d is well known in the art and is generally achieved by cross-correlating the received seismic signal against the pilot signal used to drive the vibrator.
A variety of correlation techniques are well known in the art. For example, various correlation techniques are well described by N. A. Anstey xe2x80x9cCorrelation Techniquesxe2x80x94A Reviewxe2x80x9d, Geophysical Prospecting, Vol. XII, No. 4, 1964. In simplified terms, correlation techniques identify the presence of a seismic reflection in the time domain based on the sum obtained by solving the well known cross-correlation equation:             φ              g        ⁢                  xe2x80x83                ⁢        r              ⁡          (      τ      )        =            1      T        ⁢                  ∫        0        T            ⁢                        r          ⁡                      (            t            )                          ⁢                  g          ⁡                      (                          t              -              τ                        )                          ⁢                  xe2x80x83                ⁢                  ⅆ          t                    
where g(txe2x88x92xcfx84) is the input pilot signal delayed by an amount (xcfx84) and where r(t) is the measured reflection signal. In effect, the pilot signal is xe2x80x9coverlainxe2x80x9d onto the reflection signal at incremental delay times xcfx84. At each delay time the integrated area under the signal-pilot product curve is the xe2x80x9cpulse compressedxe2x80x9d signal. The delay time equals the travel time of the reflected signal. Assuming that the wave velocities in the subsurface strata are known, the depth of the associated reflecting subsurface strata interface can be determined.
Recently, new methods of pulse compression have been developed which employ inversion rather than correlation to compress the reflected seismic signals. Examples of such inversion techniques are well described in Walker, U.S. Pat. No. 5,901,112 and Allen, U.S. Pat. No. 5,790,473.
A significant problem with conventional systems employing a vibrating baseplate to impart seismic waves into the earth is that the actual motion of the baseplate, and thus the actual seismic energy imparted to the earth, is different from the ideal motion represented by the pilot signal. This difference can be caused by a variety of factors, including (1) harmonics of the baseplate, (2) decoupling of the baseplate from the earth""s surface, (3) nonlinearities in the mechanical hydraulic system, (4) flexure of the baseplate, and (5) flexure of the frame supporting the land vibrator. The differences between the pilot signal and the actual baseplate motion are problematic because, in the past, the pilot signal was used to pulse-compress the reflected seismic signal either through correlation or inversion. Thus, if the actual motion of the baseplate was different from the ideal motion corresponding to the pilot signal, the pulse-compressed reflected seismic signal produced by correlation or inversion was inaccurate.
Various methods of obtaining a signal representative of the actual seismic energy imparted to the earth have been attempted in the past. Most of the past methods employ one or more accelerometers attached to a surface of the baseplate for providing a signal representative of the acceleration of the baseplate. This signal was then processed to obtain a signal simulating actual baseplate motion.
The use of accelerometers for measuring baseplate motion has a number of drawbacks. The primary drawback is the near impossibility of creating an accelerometer which is durable enough to withstand the conditions of the baseplate, yet sensitive enough to effectively read the vibrations of the baseplate. Typically, the most sensitive accelerometers wear out quickly when exposed to the conditions of the vibrating baseplate.
In addition, accelerometers attached to the surface of the baseplate can be physically damaged by the external environment when the seismic survey is performed on rough terrain.
It is an object of the present invention to provide a system for accurately measuring seismic energy imparted to the earth by a vibrating baseplate.
It is another object of the present invention to provide a system for more accurately, compressing a received seismic signal.
It is a further object of this invention to provide a system for effectively utilizing measurements of baseplate harmonics in generating a pulse-compressed measured seismic signal.
In accordance with the embodiment of the present invention, there is provided an apparatus for performing seismic surveys. The apparatus includes a land vibrator operable to induce seismic waves into the earth. The apparatus also includes a non-contact vibrometer for measuring the induced seismic waves.
In another embodiment of this invention, a method for conducting a seismic survey is provided. The method includes the step of vibrating a baseplate coupled to the earth to thereby induce seismic waves in the earth. The method further includes the step of measuring the induced seismic waves using a non-contact vibrometer.
The method and apparatus of this invention provide a more accurate and durable system for measuring the actual seismic energy imparted to the earth. Thus, the present invention produces more detailed and reliable seismic surveys.
Additional objects and advantages of this invention will be apparent in the description which follows and in the appended claims and drawings.